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Key Slides

2.
Air pressure Warm air is lighter Cold air is heavier The temperature of the air will influence its weight and therefore the pressure it exerts on the ground. Differences in pressure create winds, flowing from high to low pressure. Some regions of the world experiences higher pressure than expected due to the global circulation. The tropics are hot, therefore low pressure might be expected, but these regions lie under a des ending flow of air from the upper part of the troposphere which creates a high pressure belt.

3.
The trade winds were discovered by Christopher Columbus. For centuries, as the ocean explorers who followed Columbus ventured further from the civilisations of Europe they came to learn of the strong, steady winds that would push them towards their destinations in the New World. As they headed south away from Europe they learned to pick up the easterly winds that became known as the trade winds , which drew them towards the Caribbean, the Gulf of Mexico and South America. trade winds Or, if they were further south they would ride on the steady westerly winds of the higher latitudes that would carry them on around the Cape of Good Hope to the east. The return from the Americas would rely on the steady westerly winds of the North Atlantic . These winds were the constants of the globe – stronger or weaker, they would almost always be there. But why? It was a US Navy officer called Ferrel who worked out that a simple model of a continuous cycle of air from equator to pole and back could not be correct.

4.
What Ferrel proposed was a tricellular system. Taking the northern hemisphere on its own, the mass of warm air high above the equator moves north, converging on the pole, but it does not reach there. As it moves north, the narrowing of the globe means the air is squeezed together so that it sinks down to the surface, at the horse latitude of 30 degrees or so. From there, some of the air moves south again to create the trade winds , deflected to the right by the Earth’s rotation ( Coriolis effect ) to become the north-easterly trade winds . But from the horse latitude some of the air moves north at the surface, again deflected to the right by the Coriolis effect, and thus creates the reliable westerlies .

5.
The pattern of Rossby waves varies seasonally , with four to six waves in the summer and three in winter, in a continuous belt around the globe in mid-latitudes . Within the upper westerly wind belts are narrow bands of extremely fast moving air called jet streams , which flow at speeds varying between a mean of 70mph in summer and 115mph in winter, although speeds of 230mph+ have been recorded (faster in winter because there is a greater temperature difference). The polar front jet stream is the key to understanding changing surface weather because it controls the formation, survival and decay of lower-level weather systems .

6.
Rossby wave air is slower as it bends towards pole , excess air piles up because it is constricted . The convergence leads to some air being forced down, spiralling clockwise downwards to cause high pressure ( anticyclone ) at the surface. Rossby wave air faster as it turns away from pole, as it diverges and spreads . Air spirals upwards anticlockwise producing low pressure ( depression ) at the surface. The Coriolis effect spins the jet stream round the globe, west to east. It moves north and south too, following the boundary between warmer and colder air. These boundaries are also where weather fronts generally develop.

7.
Things to remember: Jet stream : belt of upper-air westerly winds which occurs vertically above the polar front. Rossby waves : the meandering paths that the PFJS follows. High pressure : caused by Rossby wave air bending towards the pole and converging, leading to some air being forced down clockwise. Low pressure : caused as Rossby wave air turns away from pole and diverges, allowing air to spiral anticlockwise upwards. Rossby waves therefore create alternate areas of high and low pressure.